Online Reference Limitation Method of Shunt-Connected Converters to the Grid to Avoid Exceeding Voltage and Current Limits Under Unbalanced Operation—Part I: Theory

Abstract

This paper focuses the analysis around a gridconnected converter that operates under unbalanced voltage and current conditions. Two different scenarios have been selected to validate the proposed algorithms. In the first scenario, the converter operates as a reactive power compensator, exchanging reactive power with the grid, and minimizing the dc bus voltage oscillations. The second scenario is more challenging, since it operates as a load balancer. In this application, the unbalanced current consumption of the load is compensated by the converter, exchanging positive and negative sequence currents with the grid. Another objective in this second scenario is to compensate the reactive power of the unbalanced load. It is important to highlight that in both scenarios, any maximum limit of the converter can be exceeded due to the specific unbalanced voltage and current conditions. Three variables are considered critical for the converter: 1) output ac current limit; 2) output ac voltage limit; and 3) dc bus voltage oscillation limit. Thus, this paper proposes a control method, which limits online the reactive power reference in the first scenario and the exchanged current references in the second scenario in order not to exceed any of the mentioned critical variables. The corresponding experimental results are shown in Part II of this paper so as to validate the limitation algorithm obtained by means of the mathematical analysis carried out in Part I.

A Review on Grid-Connected Converter Control for Short-Circuit Power Provision Under Grid Unbalanced Faults

“…The method proposed in [63] aims to minimize DC bus voltage oscillations, where reactive power reference is restricted considering not only current limit but also upper voltage limit and DC bus voltage oscillation limit. However, these methods are only applicable if active power reference is set to zero and the discontinuity in (13) caused by |v + | = |v − | for constant reactive power strategy is not addressed.…”

A Flexible Experimental Laboratory for Distributed Generation Networks Based on Power Inverters

“…Its aim is to extract the maximum allowable energy from the source [47,55]. Active power curtailment can also be required to protect the inverter from over currents when a voltage sag occurs [10,11]. When the source is dispatchable the reference active power can be set by a higher level controller.…”

“…The reactive power reference for each node can also be determined by a centralized controller when a general voltage control is required [56,57]. In addition, the balancing factors in Equations (9) and (10) can be used to provide ancillary functions to the DGS specially during voltage disturbances [10,58,59]: avoid DC-link voltage oscillation during network voltage unbalancing, provide some voltage support during unbalanced voltage droops, among others. Some degrees of freedom exist in the selection of the balancing parameters.…”

“…Some degrees of freedom exist in the selection of the balancing parameters. For instance, in this work, two additional ancillary objectives will be defined for the network-feeding nodes working with the control scheme based on Equations (9) and (10). First objective (reduction of DC-link oscillations) will be fulfilled choosing the following relationships between the sequence balancing parameters:…”

“…Thus, grid codes and standards propose different protocols for DGS during disturbances, mainly related to low-voltage ride-through (LVRT) and reactive current injection (RCI) [5,6]. In grid codes, the stringent requirements on LVRT and RCI during voltage sags [9] are a promising and profitable research field that must be taken into account [10][11][12][13][14].…”